Strings

Strings#

image.png

Strings#

char *p = "abcde" vs. char s[] = "abcde"

image.png

Example 1#

char *p = “abcde”
printf( “%s %s\n”, p, p +1 );

char s[] = “abcde”
/*	is equivalent to: */
char s[] = {‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘\0’};

#include <stdio.h>

int main() {
    char *p = "abcde";
    printf("%s\n", p); /* Prints: abcde*/
    p[0] = 'x';      /* Undefined behavior! Segmentation fault.*/
    printf("%s\n", p);
    return 0;
}

#include <stdio.h>

int main() {
    char s[] = "abcde";
    printf("%s\n", s); // Prints: abcde
    s[0] = 'x';        // Modifies the array.
    printf("%s\n", s); // Prints: xbcde
    return 0;
}

# @title
%%writefile tmp.c
#include <stdio.h>
#include <ctype.h>

int word_cnt(char *s);

int main()
{
    char str[100];

    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    int count = word_cnt(str);

    printf("The string contains %d words.\n", count);

    return 0;
}

int word_cnt(char *s)
{
    int cnt = 0;
    while (*s != '\0')
    {
        while (isspace(*s))
            ++s;
        if (*s != '\0')
        {
            ++cnt;
            while (!isspace(*s) && *s != '\0')
                ++s;
        }
    }
    return cnt;
}

  Cell In[1], line 6
    int word_cnt(char *s);
        ^
SyntaxError: invalid syntax

# @title
!gcc tmp.c -o tmp
!./tmp

Strings in the standard library#

The <string.h> library provides functions for handling and manipulating strings in C.

#include <string.h>

char *strcat(char *s1, const char *s2);

int strcmp(const char *s1, const char *s2);

char *strcpy(char *s1, const char *s2);

unsigned strlen(const char *s);

An implementation example (1)#

unsigned strlen(const char *s)
{
    int n = 0;

    for (; *s != '\0'; ++s)
        ++n;
    return n;
}

# @title
%%writefile tmp.c
#include <stdio.h>
#include <string.h>
int main()
{

    char s[] = "Hello";
    unsigned len = strlen(s);
    printf("The length of the string is: %u\n", len);
    return 0;
}

# @title
!gcc tmp.c -o tmp
!./tmp

An implementation example (2)#

char *strcat(char *s1, const char *s2)
{
    char *p = s1;

    while (*p)
        ++p;

    while (*p++ = *s2++);

    return s1;
}

Why Return s1?

String permutations#

Goal: print all permutations of a given string.

Example: “ABC” -> ABC,ACB,BAC,BCA,CAB,CBA

Can you think of a recursive solution?

image.png

Implementation (swap)#

void swap(char *x, char *y)
{
    char temp;
    temp = *x;
    *x = *y;
    *y = temp;
}

Implementation (recursion)#

void permute(char *a, int l, int r)
{
    int i;
    if (l == r)
        printf("%s\n", a);
    else
    {
        for (i = l; i <= r; i++)
        {
            swap((a + l), (a + i));
            permute(a, l + 1, r);
            swap((a + l), (a + i)); // backtrack
        }
    }
}

Ragged Arrays#

    char a[2][15] = {"abc:", "a is for apple"};
    char *p[2] = {"abc:", "a is for apple"};

    printf("%c%c%c %s %s\n%c%c%c %s %s\n",
           a[0][0],
           a[0][1], a[0][2], a[0], a[1], p[0][0], p[0][1], p[0][2], p[0], p[1]);


# @title
%%writefile tmp.c
#include <stdio.h>
int main(void)
{
    char a[2][15] = {"abc:", "a is for apple"};
    char *p[2] = {"abc:", "a is for apple"};

    printf("%c%c%c %s %s\n%c%c%c %s %s\n",
           a[0][0],
           a[0][1], a[0][2], a[0], a[1], p[0][0], p[0][1], p[0][2], p[0], p[1]);
    return 0;
}

# @title
!gcc -o tmp tmp.c
!./tmp
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